Working properly on my system.
Marked with X, hit Ctrl + Backspace keys, dialog box comes up and I select Delete from Disk. The image files are move to the Recycle bin.
Could be you have 2 different copies of those files and the ones you think are not being deleted are actually the second copy.
Go to one of the images you think are in the same folder, right click and select show in Finder/Explorer and see if it is the same folder that still has those deleted images in.
The 2 copies of the same images happens if you import from one of your drives, Not a memory card, and you forgot to select "Add" and it is set to Copy in the import dialog window.